[trentlott]

The argument is thermodynamic: meat will never provide more calories than it requires.

[TheCoelacanth]

In the case of leafy greens, most of the energy will be tied up in cellulose that will pass straight though a human digestive system undigested.

Any model that treats a human body as a perfectly efficient bomb calorimeter will produce such inaccurate results as to be virtually useless.

[toasterlovin]

Plants, as a class, have a cost per calorie with a 60x range (from about 50 to 3000 calories per $). Animals are fed with plants at the higher end of that range. If you replace your meat consumption with plants at the lower end of that scale, it does not result in lower resource consumption.